A charge blocking layer containing dispersed grain shaped titanium oxide (TiO.sub.2) particles is known. Such a blocking layer is problematic in certain situations because at low temperatures (e.g., about 10 to 15 degrees C), charge transport through the blocking layer containing the grain shaped titanium oxide particles is not efficient enough and high residual voltage (e.g., around 100 V) is observed. The present inventors have found that this low temperature problem can be reduced by increasing the concentration of the grain shaped titanium oxide particles to a high level such as over about 90% by weight based on the weight of the blocking layer. At such a high concentration, however, the parameters of dispersion quality, particle size distribution, and degree of particle settling become much worse. In fact, such a highly concentrated dispersion of grain shaped titanium oxide particles in a binder generally is unsuitable for use in dip coating the charge blocking dispersion onto a photoreceptor substrate. There is a need, addressed by the present invention, for an improved charge blocking composition that solves the above low temperature problem and that exhibits good dispersion qualities to allow dip coating of the charge blocking composition onto a photoreceptor substrate.
Conventional photoreceptors and their materials are disclosed in Katayama et al., U.S. Pat. No. 5,489,496 (discloses in col. 4, lines 57-58 that "needle-like titanium oxide particles may be used together with titanium oxide having a grain-like shape" in an undercoat layer); Yashiki, U.S. Pat. No. 4,579,801; Yashiki, U.S. Pat. No. 4,518,669; Seki et al., U.S. Pat. No. 4,775,605; Kawahara, U.S. Pat. No. 5,656,407; Markovics et al., U.S. Pat. No. 5,641,599; Monbaliu et al., U.S. Pat. No. 5,344,734; Terrell et al., U.S. Pat. No. 5,721,080; and Yoshihara, U.S. Pat. No. 5,017,449.
Conventional charge blocking layers are also disclosed in U.S. Pat. No. 4,464,450; U.S. Pat. No. 5,449,573; U.S. Pat. No. 5,385,796; and Obinata et al, U.S. Pat. No. 5,928,824.